Interclavicle

An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one.[1] In therians, it is replaced by the sternum which is similar in shape and function but forms via endochondral ossification (cartilage forming bone). The interclavicle, on the other hand, develops through intramembranous ossification of the skin. Monotremes, although part of the mammalian class, do have interclavicles. Dinosaurs also lack an interclavicle, allowing the development of sternal plates eventually culminating in the evolution of the bird sternum, a crucial step in the evolution of bird flight.

The rod-like interclavicle of Ophiacodon (a Permian synapsid) connected to a pair of rounded clavicles. Seen in ventral view (i.e. looking towards the chest from below)